Publications

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X-ray spectra and images from Al (with 5% of Mg and some with 5% of NaF dopants) and Cu (pure and with 4% of Ni) wire arrays and X-pinches were accumulated in experiments on the 1 MA pulsed power generator at UNR. In particular, axially and radially resolved K-shell X-ray spectra of Al, Mg, and Na and L-shell X-ray spectra of Cu and Ni were recorded by a KAP crystal (in a spectral region from 6 to 15 Aring) through different slits from 50 mum to 3 mm. In addition, spatially integrated harder X-ray spectra were monitored by a LiF crystal. Non-LTE kinetic models of Al, Mg, and Na, and of Cu and Ni provided spatially resolved electron temperatures and densities for experiments with Al and Cu loads, respectively. Advantages of using alloys and dopants with small concentrations for spectroscopic plasma diagnostics will be presented. Dependence of the plasma's spatial structures, temperatures, and densities from wire material and load configurations, sizes, and masses will be discussed .

Pulsed power driven metallic wire-array Z pinches are the most powerful and efficient laboratory x-ray sources. Furthermore, under certain conditions the soft x-ray energy radiated in a 5 ns pulse at stagnation can exceed the estimated kinetic energy of the radial implosion phase by a factor of 3 to 4. A theoretical model is developed here to explain this, allowing the rapid conversion of magnetic energy to a very high ion temperature plasma through the generation of fine scale, fast-growing m=0 interchange MHD instabilities at stagnation. These saturate nonlinearly and provide associated ion viscous heating. Next the ion energy is transferred by equipartition to the electrons and thus to soft x-ray radiation. Recent time-resolved iron spectra at Sandia confirm an ion temperature T{sub i} of over 200 keV (2 x 10{sup 9} degrees), as predicted by theory. These are believed to be record temperatures for a magnetically confined plasma.

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The impact of 3D structure on wire array z-pinch dynamics is a topic of current interest, and has been studied by the controlled seeding of wire perturbations. First, Al wires were etched at Sandia, creating 20% radial perturbations with variable axial wavelength. Observations of magnetic bubble formation in the etched regions during experiments on the MAGPIE accelerator are discussed and compared to 3D MHD modeling. Second, thin NaF coatings of 1 mm axial extent were deposited on Al wires and fielded on the Zebra accelerator. Little or no axial transport of the NaF spectroscopic dopant was observed in spatially resolved K-shell spectra, which places constraints on particle diffusivity in dense z-pinch plasmas. Finally, technology development for seeding perturbations is discussed.

Recent 3D hybrid simulation of a plasma current-carrying column revealed two regimes of sausage and kink instability development. In the first regime, with small Hall parameter, development of instabilities leads to appearance of large-scale axial perturbations and eventually to the bending of the plasma column. In the second regime, with five times larger Hall parameter, small-scale perturbations dominated and no bending of the plasma column was observed. Simulation results are compared to recent experimental data, including laser probing, x-ray spectroscopy and time-gated x-ray imaging during wire array implosions at NTF.

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Over the last few years, a variety of experiments studying higher photon energy (>4 keV) radiators have been performed, primarily at the Z accelerator. In this paper, the results of experiments designed to study the effects of initial load diameter on the radiated output of stainless steel wire arrays are presented. Stainless steel is primarily iron, which radiates in the K-shell at 6.7 keV. Nested wire arrays from 45 mm initial outer diameter to 80 mm outer diameter were fielded at the Z accelerator. A nested array consists of two wire arrays, with the inner concentric to an outer. All of the arrays fielded for this work had a 2:1 mass and diameter ratio (outer:inner), and the arrays were designed to have the same implosion time. A degradation of K-shell output was observed (pulse shape and power) for the smallest and largest diameter arrays, suggesting a region in which optimal conditions exist for K-shell output. The degradation at small diameters results from the reduced eta value, due to low implosion velocity. Eta is defined as the kinetic energy per ion divided by the energy required to get to the K-shell. At large diameters, a dramatic degradation of output is observed not just for the K-shell, but also for the lower energy X-rays. This may be the result of the low mass required to maintain an appropriate implosion time - there simply aren't many radiators available to participate. One other possibility is that the higher acceleration necessary at large diameters to achieve the same implosion time results in additional instability growth. Also necessary to consider are the effects of interwire gap: due to the limited wire sizes available, the interwire gap on the large diameter loads is large, in one case more than 3 mm. Comparisons of the trends observed in the experiments (radiated yield, pulse shape, and spectra) will be made to calculations previously benchmarked to K-shell data obtained at Z. The reproducibility of the arrays, advanced imaging diagnostics fielded, current diagnostics, and sensitivities of the calculations are also discussed.

A technique for manufacturing wires with imposed modulation in radius with axial wavelengths as short as 1 mm is presented. Extruded aluminum 5056 with 15 {micro}m diameter was masked and chemically etched to reduce the radius by {approx}20% in selected regions. Characterized by scanning electron microscopy, the modulation in radius is a step function with a {approx}10 {micro}m wide conical transition between thick and thin segments, with some pitting in etched regions. Techniques for mounting and aligning these wires in arrays for fast z-pinch experiments will be discussed. Axially mass-modulated wire arrays of this type will allow the study of seeded Rayleigh-Taylor instabilities in z pinches, corona formation, wire initiation with varying current density in the wire core, and correlation of perturbations between adjacent wires. This tool will support magnetohydrodynamics code validation in complex three-dimensional geometries, and perhaps x-ray pulse shaping.

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